There is a problem of formation damage caused by a drilling mud (or a flush liquid) especially for long horizontal wells since most of them are completed uncased, i.e., without a cemented and perforated flow string.
Drilling muds are complex mixtures of clay, fine particles (sized from several millimeters to less than one micron) and organic additives (polymers, surfactants etc.) contained in a “carrying” liquid—a drilling mud “base” such as water, oil or some synthetic liquid.
During a drilling process, a drilling mud filtrate with fines and clay contained therein penetrate a formation near a wellbore area. The formation near the wellbore area is under an excessive pressure and this causes a signifcant reduction of the permeability of the formation (to characterize this phenomenon the term “damage of the formation near-wellbore area” or just “formation damage” is used).
During a clean-up process (by means of gradual production starting) these components (e.g., fines and clay) are partially washed out of the near-wellbore area and its permeability partially recovers. However, a part of the components remains in pore space of the formation (absorbed on a pore surface, captured by pore throats etc.), which results in significant difference between recovered and initial permeabilities (normally, the recovered permeability does not exceed 50-70% of the initial).
A common laboratory method for checking drilling mud quality is to direct and reverse filtration through a core sample during which dynamics of permeability reduction/recovery are measured as a function of a number of injected pore volumes of the drilling mud or oil (the latter—in case of reverse pumping simulating the cleanup process).
However, clay and other drilling mud components distribution and concentration retained in the pore space along a core sample is important information for understanding mechanism of formation damage and selection of a relevant method for improving a productivity index (minimization of a bottomhole formation zone damage). These parameters are not measured in a conventional procedure for determining drilling mud quality.
One of the most well known methods for non-destructive sample analysis is an X-ray computer tomography. Thus, U.S. Pat. No. 4,540,882 describes a method for determining a drilling mud invasion using a core X-ray computer tomography with a contrast agent addition. The first material is added to the drilling mud in order to obtain a first fluid having an effective atomic number different from an effective atomic number of connate fluids contained in a formation borehole zone. A preserved core sample is collected from the borehole for scanning by a computer axial tomographic X-ray scanner to determine attenuation coefficients at a plurality of points in a cross section of the core sample. The core sample is scanned using X-rays at first and second energies. The determined attenuation coefficients for the plurality of points located in the cross section at each energy are used to determine an atomic number image for the cross section of the core sample. The depth of invasion of the first fluid is then determined from the atomic number image, as an indicator of the depth of invasion of the drilling fluid into the core sample.
Another method is disclosed in U.S. Pat. No. 4,722,095. It is based on a high X-ray attenuation coefficient of barite widely used as a weighting agent in drilling mud. First, a mud filtrate is removed from a core sample after which pore and total volume of the core sample as well as the volume of barite particles that penetrated the sample are measured using X-ray computer tomography.
Unfortunately, the use of barite as a contrast agent to evaluate the drilling mud penetration depth is not always justified because the size of these particles is comparable with the size of pore throats and, consequently, most of them will be captured in small pores near the sample inlet.
Other drilling mud components (clay, polymers, water etc.) have a weak X-ray contrast and cannot have spatial definition with the required accuracy.
The use of a contrast agent soluble in a “carrying fluid,” as it was described in U.S. Pat. No. 5,027,379, does not enable to evaluate penetration depth as well as concentration of clay and other mildly-contrasting additives contained in the drilling mud because the penetration depth of the drilling mud filtrate and the said additives is different.